1. Field of the Invention
The present invention relates to a method for local wafer thinning and reinforcement of the thinned area, and to wafers so fabricated.
2. Description of the Related Art
In the art of integrated circuits (IC's) and wafer testing it is known to use various optical microscopes to illuminate the device under test (DUT) and/or detect optical reflection or emission from the DUT. Modern testing procedures perform the testing from the backside of the DUT, which is mostly silicon having thickness of several hundreds of microns, depending on the particular DUT. Since doped silicon generally absorbs photon illumination, it is very difficult to image the DUT through a thick doped silicon layer. Therefore, it was suggested to thin the wafer down to about 100 microns for observation. On the other hand, thinning the entire wafer to 100 micron would drastically reduce its physical integrity and will probably cause breakage. Therefore, it was previously suggested to thin only the area of interest and keep the rest of the wafer at its original thickness.
FIG. 1 depicts a conventional semiconductor wafer 100 from what is generally referred to as the front side, i.e., the side upon which IC fabrication takes place. For orientation purposes, a notch 130 is provided at the bottom of the wafer. As is known, the wafer is conventionally processed to form multiple IC's, generally referred to as dies, 110, which are separated by scribe lines 120. Once processing of the wafer is completed, the scribe lines are used to separate the dies so as to provide separate IC's. The individual IC's are then encapsulated and are provided with contacts to enable communication with the circuit in which the IC is used. In FIG. 1, die 125 is shaded to indicate that die 125 is the one selected for test.
As can be appreciated, locally thinning the area of die 125 can significantly weaken wafer 100 and can lead to breakage. Notably, applying even a small pressure on die 125 can cause the wafer to break along scribe lines 120. However, many testing procedures include contacting the die 125 with a probe, which exerts pressure on the die. In the prior art it has been suggested to locally thin the wafer and then glue a glass insert inside the thinned area, so as to prevent breakage of the wafer during testing. However, the index of refraction of glass is about 1.4-1.6, while the index of refraction of silicon is about 3.6. Consequently, using such an insert causes optical aberration. Moreover, the adhesive used to glue the insert may introduce additional aberrations—depending on its index of refraction. While the adhesive may be selected so its index of refraction matches that of the glass insert, the combination would still introduce aberrations since the glass insert and adhesive will not match to the doped silicon of the DUT. Accordingly, an improved method for thinning the area of a die, while maintaining the integrity of the wafer is needed.